Fly ash is one of the residues created during the combustion of coal in coal-fired power plants. Fine particles rise with flue gasses and are collected by suitable means with filter bags or electrostatic precipitators.
Because fly ash is a by-product material of coal combustion, which is highly variable in composition, the chemistry of fly ash from various sources can vary considerably, but all fly will contain silicon dioxide (SiO2), calcium oxide (CaO), iron (III) oxide (FeO2) and aluminum oxide (Al2O3). And, depending on its source, coal may include one or more toxic chemicals, such as arsenic, beryllium, boron, cadmium, chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, in trace amounts. Mercury is a significant environmental problem and, though apparently trapped in the fly ash, can leach into the surrounding environment when ordinary use, storage or disposal measures are involved.
Coal ash can be disposed of as a wet residue in ash ponds or as a dry residue that can be sold or buried. Ash ponds are the most common form of disposal but add costs and are subject to strict regulation. Fly ash leachate can contain mercury and other heavy metals in excess of that permitted under current RCRA standards. Water flow through a fly ash pond into ground water can be controlled by using low-permeability clay layers and the like, but these add expenses and a better remedy would be desired.
Dry disposal can also be expensive. It is desirable, where possible, to dispose of the ash by selling it for use in concrete where it provides a valuable partial replacement for Portland cement. Having the property of reacting with calcium hydroxide, fly ash is useful in construction as is and can save costs associated with obtaining the naturally-occurring pozzolans traditionally used for making cement. The use of fly ash has been advocated as providing an economical way to conserve coal and partially recycle the byproducts of its combustion. It is credited with saving coal and reducing its pollution potential by reducing the amount of Portland cement that must be made, the production of which uses large amounts of coal in a type of combustion that makes it expensive to treat for pollutants. Unfortunately, the mercury and other heavy metals are too easily released to the surroundings.
Carbon content is an important factor in ash quality, and the ASTM (American Society for Testing and Materials) has set standard C618 for building codes with a limit of 6% carbon while industry preferences are seen as 3% and below. Carbon can interfere with air entrainment, which is important to controlling pouring properties. High carbon contents can also be a problem where combustion air is reduced to control NOx and where effluents are treated with activated carbon to remove mercury. Thus, efforts to control mercury by absorbing it on activated carbon has led to reduced air entraining capacity of the fly ash and limited its value for use in concrete.
The problems with emissions of hydrochloric acid and mercury have not been adequately addressed by many technologies used to reduce them and those technologies can adversely affect the quality of the ash. If not suitable for sale, the ash must be disposed of at significant expense. The environmental impact of contaminated ash has recently taken on technical and regulatory momentum.
It would be important to the environment if fly ash could be produced through the use of a group of non-sorbent chemicals which are effective for HCl and/or mercury reduction and could yield an ash product having good properties, including lowered solubility of mercury in the ash and low carbon contents. Such an improvement could have significant implications, especially because some procedures for reducing the harmful effects of HCl and/or mercury have been implicated in fly ash of reduced quality and/or ash disposal difficulties.
Accordingly, there is a present need for improved fly ash compositions and processes that can produce them while being effective at reducing HCl and/or mercury emissions from combustion gas streams by chemically converting removed chloride and mercury to chemically stable forms.